Container transport semi-trailers, while in use, must be in compliance with the relevant regulations and provisions. As for the aspect of cargo load (weight), specifically, they need to meet the requirements of a maximum cargo load limit and an axle-load limit. When a cargo is loaded, the total weight of the cargo is controllable, however, it is difficult to distribute the loaded cargo uniformly. This means that although the total weight of the cargo may be controlled effectively to meet the requirement of the maximum total weight, the non-uniformity of cargo distribution, e.g. the gravity center of the cargo biasing towards the semi-trailer axle/tractor axle, may cause the axle-load of the semi-trailer axle/tractor axle exceed the limit provided by the regulations. On the other hand, in the U.S., the total weight of load and the wheelbase are related, that is, the longer the wheelbase is, the larger the total weight is allowed, furthermore, different states in the U.S. have different provisions for the axle-load limit. Accordingly, the adjustment to the axle-load is achieved by adjusting the trailer body to change the wheelbase, in order to meet the requirement of the axle-load limit.
FIG. 1 illustrates a conventional container transport semi-trailer comprising a frame 1, a suspension 2, an axle 3 and a tractor 4 and so on, while the reference sign 5 indicates the gravity center of the cargo. In order to avoid the axle-load from being overcharged and the total weight from being changed, the most effective way is to adjust the wheelbase 6 of the semi-trailer.
In normal conditions, the position of the semi-trailer axle 3 is fixed, which means the wheelbase 6 is constant, and the suspension in such a condition is directly fixed to longitudinally extending members of the frame. Referring to FIG. 2, in order to make the wheelbase 6 adjustable, generally, the suspension 2 is fixed on a trailer body 8 with pins 10 disposed at front and rear ends of the trailer body 8, and a set of locating-hole plates 9 are fixed on the longitudinally extending members 7 of the frame. Once the trailer body 8 is moved to the desired position, the pins 10 are inserted into corresponding holes in the locating-hole plate 9, and thus the adjustment and fixation of the wheelbase are implemented.
FIGS. 3(a)-3(b) illustrates a prior locking pin mechanism for locking a slidable trailer body and longitudinally extending members of a frame, which is disclosed by U.S. Pat. No. 3,372,946 (published on March, 1968 in the name of Hutchens). The locking pin mechanism comprises a locating-hole plate 9, pins 10, pressure springs 11, a lock case 12, a linkage 13, a shaft 14, a slewing arm 15, a return spring 16 and a pull rod 17. Its operating principle is that in the working position, the pins 10 are pressed into the locating plate holes 9 by the action of the pressure springs 11, and the pull rod 17 is pulled towards left by the action of the return spring 16. At this time, the movable trailer body 8 and the longitudinally extending members 7 are coupled into one piece by the pins 10 and cannot move with respect to each other, that is the working position. When the wheelbase 6 is required to be adjusted, the pull rod 17 is firstly pulled outwardly to drive the slewing arm 15 to rotate counterclockwise, and then the linkage 13 is driven to move inwardly so to pull the pins 10 out of the locating plate holes 9, thus the separation of the trailer bodys 8 and the longitudinally extending members 7 is accomplished. At this time, the trailer bodys can move back and forth longitudinally along the direction of the longitudinally extending members, so as to perform adjustment to the wheelbase. However, in the practical operation, it is difficult for the axles of the pins 10 to be concentric with the centers of the locating plate holes 9, which means that the pins 10 and the locating plate holes 9 get stuck, thus it is difficult to move the pull rod 17 when pulling it outwardly. In order to allow the pins 10 to be pulled out of the locating plate holes 9, a generally accepted method is that one person pulls the pull rod outwards while another person forcibly moves the vehicle back and forth. Once the axles of the pins 10 are concentric with the axles of the locating plate holes 9, that is, the pins and the holes are not getting stucked any more, the pins 10 may be pulled out of the locating plate holes 9 by the action of the pulling force, and thus the separation of the trailer bodys 8 and the longitudinally extending members 7 is performed. In this situation, two persons are necessary for carrying out this operation. After the adjustment of the wheelbase is finished, the pull rod 17 is released by the operator, and pulled to the left by the return spring 16, thereby the slewing arm 15 rotates clockwise and all the linkages 13 protrude outwardly, and the pins 10 are pressed towards the locating plate hole 9 by the pressure springs 11. Generally, it is also difficult for the axles of the adjusted pins 10 to be concentric with the centers of the locating plate holes 9, and the operator also has to forcibly move the vehicle back and forth, till the centerlines of both the pins 10 and the respective locating plate holes 9 substantially align, then the pins 10 is automatically inserted into the locating plate holes 9 due to the action of the pressure springs 11, and in this situation only one operator is needed.
In fact, it is impractical for carrying out the operation with two persons, since in many cases there is only one driver driving the vehicle, and thus the operation typically could only be carried out by a single person. Therefore, improvements to the above-described structure are needed in order to allow the above-described operation to be implemented by only one person.
In view of this problem presented in the above patent, U.S. Pat. No. 4,838,566 (published on June 1989 and assigned to Holland-Binkley Corporation) discloses a structure as shown in FIGS. 4(A)-4(B). An important improvement of this patent is that one torsional spring 18 is provided at an end of the shaft 14, and the function thereof is that if the pins 10 get stuck in the locating plate holes 9 when the pull rod 17 is pulled, the slewing arm 15 is still able to rotate, and at this time, the slewing arm 15 would drive one end of the torsional spring 18 to rotate. After the operator has pulled the pull rod 17 in place completely and fixed them, the operator could then give a push to the semi-trailer to remove the stuck between the pins 10 and the locating plate holes 9, to allow the other end of the torsional spring to rotate, so the pins 10 may be pulled out of the locating plate holes 9. However, the torsional spring 18 is required to have such a rigidity that is sufficient for overcoming both the expansion force of the pressure springs 11 and the resistance of the pins 10, so as to pull the pins 10 out of the locating plate holes 9. Moreover, when the pins get stuck, the pins 10 could be pulled out of the locating plate holes 9 only when the stuck all the pins 10 are released from lock simultaneously. Similarly, after the adjustment of the wheelbase is finished, the operator releases the pull rod 17, each of the pins 10 is pressed towards the respective locating plate hole 9 under the action of the pressure springs 11, and all the pins 10 could be inserted simultaneously only when the axles of these pins 10 are simultaneously aligned with the centers of the locating plate holes 9.
In view of the problems presented in the above patents, a new U.S. Pat. No. 5,480,171 (Published on January 1996 and assigned to Hutchens Industries, Inc.) provides a complete solution, the key thereof is that a new improvement is performed on the locking pin structure. FIG. 5 illustrates the working principle of this patent. Springs 64 of a relative low stiffness coefficient are provided on the inside end of the pins 58, and springs 68 of a relative high stiffness coefficient are provided on the end of linkages 72 adjacent to the locking pins. The springs 64 are restricted at one end by a fixing plate 44, and are restricted at another end by stepped surfaces of inside faces of the pins 58. The springs 64 are always in a compressed state such that a force applied inwardly is produced, and the force pushes the pins 58 inwardly and causes the pins 58 to release from the locating plate holes 18. The function of the spring 68 is to press the pins 58 into the locating plate holes 18, and the pins 58 are also required to overcome the force applied by the springs 64 during pressing, hence the force produced by the springs 68 is required to be great sufficiently. Referring to FIG. 6, when the wheelbase is required to be adjusted, the operator pulls the pull rod 90 outwardly and fixes it to the position 91. The pull rod will drive the slewing arm 82 to rotate. Since the connection plate 78 and the slewing arm 82 are fixed to the shaft, when the slewing arm 82 rotates, the connection plate 78 is driven to rotate, and thus the linkages 72 are pulled to move inwardly. The other end of each of the linkages 72 then compresses the spring 68. If the pins 58 and the locating plate holes 18 are concentric with each other at this moment, the pins 58 will be pushed inwardly under the action of the spring 64, so that the pins 58 are pulled out of the locating plate holes 18, as shown on the left side of FIG. 6. If the pins 58 and the locating plate holes 18 are in the state of stuck condition, tips 70 of the pull rods 72 would separate from tips 66 of the pins 58. The operator would then give a push to the vehicle to make the pins 58 to be concentric with the locating plate holes 18, and push the pins 58 inwardly to exit the locating plate holes 18 with the action of the springs 64. After all the pins 58 have exited the locating plate holes, the operator can adjust the wheelbase. After the adjustment of the wheelbase is finished, the operator will release the pull rod 90, and by this time all the compressed springs 68 push the pins 58 to insert them into the respective locating plate holes 18. If all the axles of the pins 58 are correctly concentric with the center of the locating plate holes 18, the pins 58 would directly enter into the locating plate holes 18, otherwise, the pins 58 would get stuck outside the locating plate holes. At this time, again, the operator would give a push to the vehicle to make the axles of the pins 58 to be concentric with the center of the locating plate holes, and then the pins 58 would enter into the locating plate holes 18, so one adjustment of the wheelbase is completed.
The actions of pulling the pins 58 out of the respective locating plate holes 18 can be performed independently from each other in U.S. Pat. No. 5,480,171, so the operability of this patent is improved greatly in contrast to the U.S. Pat. No. 4,838,556. However, in practical operation, it is very laborious for the operator to pull the pull rod 90 outwardly, because the elastic forces of the springs 68, which are required to be capable of pushing the pins 58 to move while overcoming the force applied by the spring 64, are very big. Furthermore, when the operator releases the pull rod 90, the springs 68 will on the one hand compress the springs 64 and on the other hand push the pins 58 to insert them into the respective locating plate holes 18. In this procedure, if one of the pins 58 runs into obstacle in the procedure of being inserted into the locating plate hole 18, the other pins 58 cannot be inserted into the corresponding locating plate holes 18 either.
Accordingly, it is desirable to provide a locking pin mechanism which has a better operability and reliability, and can be operate easily by the operator.